The electrochemical behaviour of naturally passivated hafnium in aqueous solutions of different pH

Naturally passivated hafnium was investigated in solutions of different pH. The effect of prevailing gas on the electrochemical behaviour of the metal was studied. The steady state potential of the metal was found to be a linear function of the solution pH in the range 4–10. The slope of E_SS vs. pH relation is equal to 18 mV/pH indicating the exchange of three electrons in the electrochemical equilibrium. In oxygen saturated solutions, the potential gets more positive which reflects the increased passivity of the metal surface in naturally aerated and oxygen saturated aqueous solutions. The passive film is highly stable in neutral solutions due to the presence of the stable HfO(OH)₂. The activation energy of the passive film dissolution is relatively lower in acid solution due to the formation of the hafnyl ions HfO²⁺. EIS and polarization experiments are consistent and the naturally occurring passive film can be fitted to a simple electronic equivalent circuit. Naturally passivated hafnium deviate slightly from ideal capacitor behaviour. Anodic oxide films on hafnium deviate greatly from the ideal capacitor behaviour and complicated electronic circuits should be proposed to fit their impedance characteristics.